Field of the Disclosure
Embodiments of the present application relates to the art of a vehicle control system configured to control an internal combustion engine and an automatic transmission that changes an input speed from the internal combustion engine.
Discussion of the Related Art
In the conventional lean-burn engines, air/fuel ratio can be altered between stoichiometric ratio in which the engine is run at the stoichiometric point and lean-burn ratio in which a ratio of fuel to air is decreased to save the fuel.
However, torque drop may be caused during transition from the stoichiometric ratio to the lean-burn ratio. In order to prevent such torque drop, an idle speed of the engine is raised by increasing an opening degree of an ISC (i.e., Idle Speed Control) valve.
During execution of the above-explained idle speed control, an engine speed may be raised abruptly by a delay in a closing motion of the ISC valve when shifting a gear stage of a transmission. JP-A-2000-170586 describes an engine control device configured to retard an ignition timing of the lean-burn engine run with the lean-burn ratio during shifting of the transmission.
JP-A-11-170892 describes a vehicle control device configured to inhibit a switching of air/fuel ratio between the lean-burn ratio and the stoichiometric ratio during shifting a gear stage of the transmission, or to inhibit a shifting of the gear stage during switching the air/fuel ratio. According to the teachings of JP-A-11-170892, the control device inhibits both of the shifting operation of the gear stage and the switching operation of the air/fuel ratio when a driver intends to accelerate a vehicle.
JP-A-2003-214216 also describes a control device and a control method of the lean-burn engine. According to the teachings of JP-A-2003-214216, a switching demand of air/fuel ratio from the stoichiometric ratio to the lean-burn ratio is limited if an engine load is fluctuated by an accelerating operation, a clutch operation, or a speed change operation.
During the lean-burn operation of the engine, since the ratio of the fuel to the air is decreased, an engine misfire may be caused as a result of ignition retard. In order to prevent such engine misfire, a conventional torque-down control may be effective. However, the torque-down control may result in a delayed response of the vehicle.